1. Field of the Invention
The present invention generally belongs to the technical field of cellular mobile communications, and particularly relates to a connection destination base station determination device that determines an appropriate connection destination radio base station from among multiple radio base stations.
2. Description of the Related Art
A mobile terminal in a cellular communications system is capable of performing communications while moving in a geographic area covered by multiple cells. Allocation of communication resources or radio resources is managed cell by cell or sector by sector. When the mobile terminal moves beyond a cell or sector, a process called handover is performed, so that radio resources to be used in a move destination cell or sector are suitably determined.
In a cellular communications system, when handover is performed, a radio base station transmitting a control signal whose received signal strength indication (RSSI) is high in a mobile terminal is selected as a move destination or connection destination radio base station.
Further, in the communications system described in Japanese Laid-Open Patent Application No. 9-200841 (Patent Document 1), a radio base station whose RSSI is greater than a predetermined value and whose number of available channels is the largest is preferentially selected as a connection destination radio base station. This enables reduction in traffic concentration on a single radio base station and traffic decentralization. It is also possible to reduce a mobile terminal's wait period for a call connection by preferentially assigning a radio base station having a larger number of available channels to the mobile terminal.
Patent Document 1
Japanese Laid-Open Patent Application No. 9-200841
By the way, the contents of communications in conventional mobile communications have been audio-centered and relatively small in data amount. However, recent developments in mobile communications technology have made it possible to transmit large amounts of data such as images and video. Therefore, the amount of data communicated by a mobile terminal may vary from small to large. Accordingly, the communications resources requested by a mobile terminal differ significantly depending on the data volume and data rate. In particular, a mobile terminal that desires to transmit a large amount of data at high speed requires a large amount of communications resources.
Consideration is given of the case where a mobile terminal requesting high-speed data transmission makes a determination based only on RSSI in determining the move destination radio base station of the handover. In this case, the radio base station that provides the highest RSSI does not necessarily possess a sufficient amount of available resources for high-speed data transmission, and without it, a connection request is rejected. Then, similarly, the amount of resources requested by the mobile terminal and the amount of available resources are also compared with respect to the radio base station providing the next highest RSSI, and thereafter, it is determined whether to authorize connection in the same manner. Accordingly, according to this method, it is not less likely that a new connection request by a mobile terminal that desires high-speed data transmission will be rejected, thus causing a problem in that it takes a long time before an appropriate connection destination is found.
On the other hand, according to the invention described in Patent Document 1, resource allocation is performed so that the rate of use of radio resources is uniform among radio base stations. Accordingly, if the rate of use becomes relatively high-level, there may be a problem in that although the total amount of available resources of the radio base stations exceeds the amount of resources requested by a mobile terminal, none of the radio base stations satisfies the amount of resources requested by the mobile terminal, so that no connection can be made to any of the radio base stations. For example, it is assumed that a mobile terminal requires radio resources for four channels and that two radio base stations possess the amounts of available resources of three channels and of two channels, respectively, at the time of handover. In this case, the amount of available resources for three+two=five channels exists in the system as a whole, and yet neither radio base station can answer the request of the mobile terminal independently. Thus, according to the conventional method, there is concern for reduction in the usage efficiency of communications resources in a communications system required to satisfy requested resources varying in amount from small to large.